Capefeather
As much as I like Birkal's idea, the problem is that most of the mediocre pokemon suggested are simply outclassed in many ways; if the "counter" beats our CAP, then it'll be hard to make it so several other things don't. If Virizion can take on something, what makes Celebi or Keldeo (who have similar typings and more passable stat spreads) not able to counter it just as well? If it beats them, why does Virizion alone take it on? Seems like it'd have to be extremely technical and specialized to even have a chance of working.